Latest runtimes and latest Unity, in a fresh blank project.
If I open the Raptor example, and (in-Editor) move the Ground up to the raptor's feet, the feet flicker but seem to be placing correctly. When I move the ground downwards the feet shoot upwards and disappear entirely.
Pressing Play brings one foot back, but not the other.
EDIT: odd behaviour seems to be limited to in-editor; moving the ground with the game playing seems to work fine :/